Gear

ABSTRACT

A gear comprised of a tooth surface and a hard carbon film formed on at least a part of the tooth surface. When the gear is used in lubricant including a specific component, a friction of a tooth surface of the gear is largely decreased, and therefore the gear performs an excellent power transmission efficiency. Further, when the gear is employed in a planetary gear mechanism or speed reducing mechanism which has a plurality of meshing portions of gears, the power transmission efficiency of the mechanism is also improved.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application has the following related applications: U.S. patentapplication Ser. Nos. 09/545,181 based on Japanese Patent ApplicationHei-11-102205 filed on Apr. 9, 1999; 10/468,713 which is the designatedstate (United States) application number of PCT Application JP02/10057based on Japanese Patent Application 2001-117680 filed on Apr. 17, 2001;10/355,099 based on Japanese Patent Application 2002-45576 filed on Feb.22, 2002; 10/682,559 based on Japanese Patent Application No.2002-302205 filed on Oct. 16, 2002; and 10/692,853 based on JapanesePatent Application 2002-322322 filed on Oct. 16, 2002.

BACKGROUND OF THE INVENTION

The present invention relates a gear for a power transmission mechanism,and more particularly to a gear which improves a transmissionefficiency.

Generally, gears have been employed in various power transmissionmechanisms. Representative gears for vehicle transmission mechanisms aremainly made by steel, such as carburized steel, carbonitrided steel, andchromium molybdenum steel.

SUMMARY OF THE INVENTION

Such gears for vehicle transmission mechanisms are required to reducefriction generated at the mesh between the gears in order to improve anoutput power and a fuel consumption of a vehicle. More specifically,there are a large number of meshes of gears in a planetary mechanism orspeed reduction mechanism, and therefore, it has been strongly desiredto decrease the frictions at the meshes of the gears in view ofimproving a power transmission efficiency of such mechanisms.

It is therefore an object of the present invention to provide animproved gear which performs an excellent power transmission efficiencyby decreasing friction on the tooth surface of the gear.

An aspect of the present invention resides in a gear which comprises atooth surface and a hard carbon film formed on at least a part of thetooth surface.

The other objects and features of this invention will become understoodfrom the following description with reference to the accompanyingdrawings.

DETAILED DESCRIPTION OF THE INVENTION

Gears are generally slidingly contacted with each other during therevolution under an engaged (meshed) state with other gear. Therefore,reducing friction at tooth surfaces of gears is preferable in view ofimproving a power transmission efficiency of the gears.

A gear of the present invention is constructed such that a hard carbon10 film (coating) is formed on at least a part of the gear. This hardcarbon film is a film of amorphous carbon or hydrogen containingamorphous carbon which is referenced as a-C:H (amorphous carbon orhydrogen containing amorphous carbon), i-C (i carbon) and DLC(diamond-like carbon).

It is known that such a hard carbon film has a low friction coefficientwhen slides on the other surface under a dry condition (no lubricatingcondition). Although the principle of having such a low frictioncoefficient has not been completely elucidated, it can be assumed that asolid lubrication performance and properties such as a low Young'smodulus and a high hardness of the hard carbon film attain the reductionof the friction coefficient.

On the other hand, when the hard carbon film is used in lubricant, thefriction coefficient of the film in lubricant becomes different fromthat in the dry condition. A normal hard carbon film is made of carbonand unavoidable impurity, or of carbon, hydrogen and unavoidableimpurity. A reactivity of a surface of the hard carbon film is low, andthis property relates to a low-friction and a low-abrasion thereof.However, since the hard carbon film has a weak interaction relative to abase oil and additives in lubricant, a reduction merit of the frictioncoefficient in lubricant is relatively small as compared with that inthe dry condition.

The present invention has been achieved by thorough study as to a hardcarbon film having a low-friction coefficient in lubricant, a preferablelubricant, and an additive component.

First, a hard carbon film is formed on a tooth surface of a gear. Thehard carbon film may be formed on a whole surface of the tooth surfaceof the gear or a partial surface of the tooth surface. Generally, thereis a tendency that a forming of a film on a bottom portion of each geartooth is not easy as compared with that on a top portion of each geartooth. However, it is not necessary to forcibly form the hard carbonfilm on the uneasy forming portion, since the merit obtained by the hardcarbon film is obtained according to a ratio of the film formed area.Further, even if a part of the film is worn out in use, the merit of thehard carbon film is maintained according to a size of the remaining hardcarbon film.

Although gears are actually used by combining two or more gears, thehard carbon film may be formed on the whole of the tooth surface or maybe formed on a part of the tooth surface. A film forming area of thehard carbon film may be properly determined upon taking account of aproduction cost, a productivity and a degree of the obtained merit.

Although a base metal of the gear is not basically limited, a carburizedsteel and a chromium molybdenum steel are preferably used to ensure animpact strength and a bending fatigue strength necessary for a gear.Further, carbonitrided steel is preferably used to suppress thesoftening of the base metal due to the semi-high-temperature conditionduring the film production process. An intermediate layer may be formedbetween the base metal and the hard carbon film to decrease the strainbetween the base metal and the hard carbon film and to improve theadherence of the film relative to the base metal. A commonly knownmethod may be employed to form the intermediate layer.

The hard carbon film can be produced by a chemical vapor deposition(CVD) process or physical vapor deposition (PVD) process. Generally, ahard carbon film formed by CVD process contains hydrogen due to rawmaterials of organic compound, and the hydrogen amount of such producedfilm ranges from 15 to 40 atom % (atomic percent). On the other hand,PVD process is capable of producing the hard carbon film with and/orwithout hydrogen. Various processes of PVD have been proposed and put inpractical use. The hard carbon film of the gear according to the presentinvention is preferable to be formed by means of an arc ion plating orspattering, in view of the adherence of the film on the base metal.

It is preferable that the hydrogen amount in the hard carbon film forthe gear is as small as possible since the decrease of the hydrogenamount in the hard carbon film decreases the friction of the gear.Therefore, the hydrogen amount of the hard carbon film of the gearaccording to the present invention is set to be smaller than 1 atom %,and preferably smaller than 0.3 atom %. That is, it is preferable thatthe hard carbon film is formed by means of physical vapor deposition(PVC) process. The hydrogen amount in the hard carbon film is capable ofbeing measured by a secondary ion mass spectroscopy (SIMS) or Rutherfordbackscattering spectroscopy (RBS).

The gear according to the present invention exhibits an excellentcharacteristic, particularly when it is used in or with lubricant. Whenthe gear according to the present invention is used in lubricant, thelubricant may be properly selected from a lubricant using mineral oil orsynthetic oil as base oil, such as gear oil, vehicle engine oil, turbineoil and spindle oil. Further, when poly-α-olefin is used as base oil ofthe lubricant, the friction decreasing merit is further improved. Thereason thereof may be thought to be that poly-α-olefin oil has aproperty of easy adherence (deposition) onto the hard carbon film formedon the tooth surface of the gear.

Further, it is preferable that a compound including hydroxy group isadded to lubricant as an additive, to further improve the frictionreducing merit. The reason thereof is guessed that the said additiveadheres on to the hard carbon film on the tooth surface of the gearthrough the hydroxy group. Further, it is preferable that the number ofthe hydroxy groups included in a molecular of the additive is as largeas possible, in view of increasing the adsorption strength. However, ifthe number of the hydroxy groups is too large, there causes apossibility that the additive is separated from the base oil due to theexcessive hydrophilicity. Therefore, the molecular structure of theadditive should be designed upon taking account of the above-discussedpoints. Further it is preferable that the molecular structure of theadditive is designed such that the hydroxy groups are located as near aspossible in the molecular structure in case that the number of thehydroxy groups in one molecular is the same. A typical molecular for theadditive is secondary alcohol (dihdyric alcohol) and tertiary alcohol(trihydric alcohol). Although the additive amount of the additive may beproperly varied according to a usage pattern of the lubricant relativeto the gear, it is preferable that the additive amount relative to thelubricant is within a range from 0.5 to 8 weight %. If the additiveamount is too small, the friction reducing merit becomes small. If toolarge, there is a possibility that the additive is separated from thebase oil.

As an additive of the lubricant, ester is preferable, and monoester ofglycerin is more preferable. It is preferable that the number of carbonatoms of fatty acid constructing glycerin monoester is greater than orequal to 8, and preferably greater than or equal to 12. If the moleculesize of the fatty acid consisting the ester in the additive is small, afilm directly formed on a surface of the hard carbon film due to theadditive becomes too thin, and therefore the friction reducing merit isdecreased thereby. Polyhydric alcohol except for glycerin may beemployed as an ingredient for the fatty ester additive of the lubricantalthough it is disadvantageous in cost.

Generally lubricant is obtained by adding proper additives in base oilsuch as mineral oil or synthetic oil. However, according to the usagecondition and the usage circumstance of gears, lubricant including ahydroxy compound as a main component may be used instead of theabove-discussed lubricant. If the lubricant including hydroxy compoundis employed, the power transmission efficiency is largely improved.

Alcohol is preferable as the above-discussed hydroxy compound, andparticularly, glycerin performs a large friction reducing effect.Further, when the gear slides in ethylene glycol, the excellent frictionreducing merit is ensured thereby subsequent to a case that glycerin isused as lubricant for gears.

It is not necessary to construct the whole of the lubricant by thehydroxy compound. In response to the request and in correspond to usagesuch as wear prevention, rust prevention, viscosity control andanti-oxidation, various known additives may be added in lubricant. Thetotal amount of such additives in lubricant is normally set to besmaller than or equal to 15 vol. %.

Further, in case that the gear according to the present invention isemployed in a mechanism having a lot of mesh portions of gears, such asa planetary gear mechanism and a speed reducing mechanism, theperformance of improving the power transmission efficient thereby isclearly ensured. In case that a planetary gear mechanism is employed ina speed reduction mechanism, it is possible to obtain a large speedreduction ratio while suppressing the size of the speed reductionmechanism. However, setting the speed reduction ratio at a large valueradically degrades the power transmission efficiency. Accordingly, byusing the gear according to the present invention as at least one of asun gear, planetary gears and a ring gear of a planetary gear mechanismfor a speed reduction mechanism, the degradation of the powertransmission efficiency in the speed reduction mechanism is suppressed.

Hereinafter, there is discussed Examples according to the presentinvention and Comparative Examples thereof.

EXAMPLE 1 and COMPARATIVE EXAMPLE 1

Chromium molybdenum steel defined as SCM420H in JIS (Japan IndustrialStandard) was employed as material of the gear of Example 1. Thematerial was machined into a gear defined by the followingspecifications, and the carburizing, quenching and tempering processeswere applied to the machined gear.

<Specifications of Gear>

Type of gear: spur gear

Module: 4 mm

Number of gear teeth: 60 (that of meshed gear: 40)

Face width: 10 mm

The above discussed gear of Example 1 was surfaced and degreased.Subsequently, a hard carbon film was formed on a tooth surface of thegear of Example 1 by arc ion plating (AIP) process. A thickness of thehard carbon film at a center portion of each tooth was 1.2 μm. Thehydrogen amount in the hard carbon film was 0.1 atom % (atomic percent)as a result of the measurement using a secondary ion mass spectroscopy(SIMS).

The tooth surface of the gear coated by the hard carbon film waspolished to remove droplets of the hard carbon film and to smoothen thesurface thereof. A surface roughness Ra of the polished tooth portionwas 0.04 μm. A gear (drive gear) meshed with the film coated gear wasnot coated with the hard carbon film. A surface roughness Ra of a toothsurface of the meshed (counter) gear was 0.17 μm. Hereinafter, as far asit is not specifically explained, tooth portions of Examples andComparative Examples were finished such that a surface roughness Ra ofthe gear coated with DLC (diamond-like carbon) ranged from 0.02 μm to0.06 μm, and a surface roughness Ra of the gear without DLC ranged from0.1 μm to 0.3 μm. Since it is difficult to further smoothen the surfaceof the gear coated with DLC by the polishing due to the property of DLCfilm, the gear coated with DLC was previously grinded and polished to besmoothed before DLC is formed on the surface of the gear. The surfaceroughness Ra is explained as Ra₇₅ in JIS (Japanese Industrial Standard)B0601(:2001).

The power transmission efficiency of the gear was measured using a powercirculation type gear test equipment. In order to separately obtain aloss of a drive gear and bearings and a loss of the tested gear, it isnecessary to execute various adaptations such as a special design of thedrive gear (counter gear) and a separate measurement of the bearingloss. However, since it is possible to determine the advantages gainedby the hard carbon film from the magnitude of the total loss withoutexecuting the separate detection of the losses of the total loss, theevaluations of Examples and Comparative Examples have been made from thetransmission efficiency corresponding to the total loss.

More specifically, using the power circulation type gear test equipment,the total loss of the tested gear was measured under a condition thatthe tested gear and the counter gear were wholly soaked inpoly-alfa(α)-olefin (PAO) oil and that the drive gear meshed with thetested gear was rotated at a speed of 6000 rpm (the revolution speed ofthe drive gear).

On the other hand, the gear of Comparative Example 1 was the same inshape and in material as that of Example 1 except that no hard carbonfilm was formed on the gear of Comparative Example 1. The total loss ofComparative Example 1 was measured under the condition as same as thatof Example 1.

EXAMPLE 2 and COMPARATIVE EXAMPLE 2

Chromium molybdenum steel defined as SCM440H in JIS was employed asmaterial of the gear of Example 2. The material was machined into a geardefined by the following specifications, and the carburizing, quenchingand tempering processes were applied to the machined gear.

<Specifications of Gear>

Type of gear: spur gear

Module: 2 mm

Number of gear teeth: 60 (that of meshed gear: 20)

Face width: 3 mm

The above discussed gear of Example 2 was degreased and set in a vacuumchamber. A hard carbon film was formed on a tooth surface of the gear ofExample 2 by arc ion plating (AIP) process in the vacuum chamber. Athickness of the hard carbon film at a center portion of each tooth was1.4 μm. The hydrogen amount in the hard carbon film was 0.1 atom % as aresult of the measurement using a secondary ion mass spectroscopy(SIMS). The tooth surface was polished to remove droplets of the hardcarbon film and to smoothen the surface. A gear (drive gear) meshed withthe film coated gear was not coated with the hard carbon film.

Using the power circulation type gear test equipment, the total loss ofthe tested gear was measured under a condition that the tested gear andthe counter gear were wholly soaked in poly-alfa-olefin (PAO) oil andthat the tested gear meshed with the drive gear was rotated at a speedof 6000 rpm (the revolution speed of the drive gear). A kineticviscosity of the employed poly-alfa-olefin was 4.0 cSt at 100° C.Hereinafter, the evaluation of examples and comparative examples wasexecuted using the poly-alfa-olefin as same as that employed in Example2.

On the other hand, the gear of Comparative Example 2 was the same inshape and in material as that of Example 2 except that no hard carbonfilm was formed on the gear of Comparative Example 2. The total loss ofComparative Example 2 was measured under the condition as same as thatof Example 2.

EXAMPLE 3 and COMPARATIVE EXAMPLE 3

Chromium molybdenum steel defined as SCM420H in JIS (Japan IndustrialStandard) was employed as material of the gear of Example 3. Thematerial was machined into a gear defined by the followingspecifications, and the carburizing, quenching and tempering processeswere applied to the machined gear. Thereafter, a finishing touch wasapplied to the processed gear.

<Specifications of Gear>

Type of gear: spur gear

Module: 6 mm

Number of gear teeth: 120 (that of meshed gear: 40)

Face width: 12 mm

The above discussed gear of Example 3 was degreased and set in a vacuumchamber. A hard carbon film was formed on a tooth surface of the gear ofExample 3 by arc ion plating (AIP) process in the vacuum chamber. Athickness of the hard carbon film at a center portion of each tooth was0.9 μm. The hydrogen amount in the film was 0.2 atom % as a result ofthe measurement using a secondary ion mass spectroscopy (SIMS). Thetooth surface was polished to remove droplets of the hard carbon filmand to smoothen the surface. A gear (drive gear) meshed with the coatedgear was not coated with the hard carbon film.

Using the power circulation type gear test equipment, the total loss ofthe tested gear was measured under a condition that the tested gear andthe counter gear were wholly soaked in poly-alfa-olefin (PAO) oil andthat the tested gear meshed with the drive gear was rotated at a speedof 9000 rpm (the revolution speed of the drive gear).

On the other hand, the gear of Comparative Example 3 was the same inshape and in material as that of Example 3 except that no hard carbonfilm is formed on the gear of Comparative Example 3. The total loss ofComparative Example 3 was measured under the condition as same as thatof Example 3.

EXAMPLE 4 and COMPARATIVE EXAMPLE 4

Chromium molybdenum steel defined as SCM420H in JIS was employed asmaterial of the gear of Example 4. The material was machined into a geardefined by the following specifications, and the carburizing, quenchingand tempering processes were applied to the machined gear. Thereafter, afinishing touch was applied to the processed gear.

<Specifications of Gear>

Type of gear: spur gear

Module: 6 mm

Number of gear teeth: 120 (that of meshed gear: 40)

Face width: 12 mm

The above discussed gear of Example 4 was degreased and set in a vacuumchamber. A hard carbon film was formed on a tooth surface of the gear ofExample 4 by the magnetron spattering process in the vacuum chamber. Athickness of the hard carbon film at a center portion of each tooth was1.3 μm. The hydrogen amount in the film was 0.1 atom % as a result ofthe measurement using a secondary ion mass spectroscopy (SIMS). Thetooth surface was polished to remove droplets of the hard carbon filmand to smoothen the surface. A gear (drive gear) meshed with the coatedgear was not coated with the hard carbon film.

Using the power circulation type gear test equipment, the total loss ofthe tested gear was measured under a condition that the tested gear andthe counter gear were wholly soaked in poly-alfa-olefin (PAO) oil andthat the tested gear meshed with the drive gear was rotated at a speedof 9000 rpm (the revolution speed of the drive gear).

On the other hand, the gear of Comparative Example 4 was the same inshape and in material as that of Example 4 except that no hard carbonfilm was formed on the gear of Comparative Example 4. The total loss ofComparative Example 4 was measured under the condition as same as thatof Example 4.

EXAMPLE 5 and COMPARATIVE EXAMPLE 5

Chromium molybdenum steel defined as SCM440H in JIS was employed asmaterial of the gear of Example 5. The material was machined into a geardefined by the following specifications, and the carburizing, quenchingand tempering processes were applied to the machined gear. Thereafter, afinishing touch was applied to the processed gear.

<Specifications of Gear>

Type of gear: spur gear

Module: 2 mm

Number of gear teeth: 60 (that of meshed gear: 20)

Face width: 3 mm

The above discussed gear of Example 5 was degreased and set in a vacuumchamber. A hard carbon film was formed on a tooth surface of the gear ofExample 5 by a plasma CVD process in the vacuum chamber. Gas employed inthe CVD process was cyclohexane. A thickness of the hard carbon film ata center portion of each tooth was 3.0 μm. The hydrogen amount in thehard carbon film was 25 atom % as a result of the measurement using asecondary ion mass spectroscopy (SIMS). The tooth surface was polishedto remove droplets of the hard carbon film and to smoothen the surface.A gear (drive gear) meshed with the coated gear was not coated with thehard carbon film.

Using the power circulation type gear test equipment, the total loss ofthe tested gear was measured under a condition that the tested gear andthe counter gear were wholly soaked in poly-alfa-olefin (PAO) oil andthat the tested gear meshed with the drive gear was rotated at a speedof 6000 rpm (the revolution speed of the drive gear).

On the other hand, the gear of Comparative Example 5 was as same inshape and in material as that of Example 5 except that no hard carbonfilm is formed on the gear of Comparative Example 5. The total loss ofComparative Example 5 was measured under the condition as same as thatof Example 5.

EXAMPLE 6 and COMPARATIVE EXAMPLE 6

Chromium molybdenum steel defined as SCM420H in JIS was employed asmaterial of the gear of Example 6. The material was machined into a geardefined by the following specifications, and the carburizing, quenchingand tempering were applied to the machined gear.

<Specifications of Gear>

Type of gear: helical gear

Module corresponding to spur gear: 4 mm

Number of gear teeth: 60 (that of meshed gear: 60)

Face width: 20 mm

Helix angle: 12°

The above discussed gear of Example 6 was surfaced (finished) anddegreased. Subsequently, the gear for Example 6 was set in a vacuumchamber, and a hard carbon film was formed on a tooth surface of thegear of Example 6 by arc ion plating (AIP) process in the vacuumchamber. A thickness of the film at a center portion of each tooth was1.0 μm. The hydrogen amount in the film was 0.1 atom % as a result ofthe measurement using a secondary ion mass spectroscopy (SIMS).

The tooth surface was polished to remove droplets of the hard carbonfilm and to smoothen the surface. A gear (drive gear) meshed with thecoated gear was not coated with the hard carbon film.

Using the power circulation type gear test equipment, the total loss ofthe tested gear was measured under a condition that the tested gear andthe counter gear were wholly soaked in poly-alfa-olefin (PAO) oil andthat the tested gear meshed with the drive gear was rotated at a speedof 6000 rpm (based on the drive gear).

On the other hand, the gear of Comparative Example 6 was as same inshape and in material as that of Example 6 except that no hard carbonfilm was formed on the gear of Comparative Example 6. The total loss ofComparative Example 6 was measured under the condition as same as thatof Example 6.

EXAMPLE 7 and COMPARATIVE EXAMPLE 7

Chromium molybdenum steel defined as SCM440H in JIS was employed asmaterial of the gear of Example 3. The material was machined into a geardefined by the following specifications, and the carburizing, quenchingand tempering processes were applied to the machined gear. Thereafter, afinishing touch was applied to the processed gear.

<Specifications of Gear>

Type of gear: spur gear

Module: 2 mm

number of gear teeth: 60 (that of meshed gear: 20)

Face width: 3 mm

The above discussed gear of Example 7 was degreased and set in a vacuumchamber. A hard carbon film was formed on a tooth surface of the gear ofExample 7 by arc ion plating (AIP) process in the vacuum chamber. Athickness of the hard carbon film at a center portion of each tooth was1.1 μm. The hydrogen amount in the film was 0.1 atom % as a result ofthe measurement using a secondary ion mass spectroscopy (SIMS). Thetooth surface was polished to remove droplets of the film and tosmoothen the surface. A gear (drive gear) meshed with the coated gearwas also coated with the hard carbon film. A thickness of the hardcarbon film at a center portion of each tooth of the drive gear was 1.0μm. The hydrogen amount in the hard carbon film of the drive gear was0.1 atom %.

Using the power circulation type gear test equipment, the total loss ofthe tested gear was measured under a condition that the tested gear andthe counter gear were wholly soaked in poly-alfa-olefin (PAO) oil andthat the tested gear meshed with the drive gear was rotated at a speedof 6000 rpm (the revolution speed of the drive gear).

On the other hand, the gear of Comparative Example 7 was as same inshape and in material as that of Example 7 except that no hard carbonfilm is formed on the gear of Comparative Example 7. The total loss ofComparative Example 7 was measured under the condition as same as thatof Example 7.

EXAMPLE 8 and COMPARATIVE EXAMPLE 8

The gears of Example 2 and Comparative Example 2 were evaluated underthe different test conditions. More specifically, lubricant employed inExample 8 and Comparative Example 8 was lubricant obtained by fullymixing an ester component which was fatty monoglyceride (a maincomponent of fatty acid is oleic acid) of 3 weight % of the total of thelubricant with poly-alfa-olefin. The other conditions of Example 8 andComparative Example 8 were the same as those of Example 2. Theevaluation of Example 8 and Comparative Example 8 were also the same asthat of Example 2.

EXAMPLE 9 and COMPARATIVE EXAMPLE 9

The gears of Example 2 and Comparative Example 2 were evaluated underthe different test conditions. More specifically, lubricant employed inExample 9 and Comparative Example 9 was lubricant obtained by fullymixing an ester component which was fatty monoglyceride (a maincomponent of fatty acid is palmitic acid) of 2 weight % in the total ofthe lubricant with poly-alfa-olefin. The other conditions of Example 9and Comparative Example 9 were the same as those of Example 2. Theevaluation of Example 9 and Comparative Example 9 were also the same asthat of Example 2.

EXAMPLE 10 and COMPARATIVE EXAMPLE 10

The gears of Example 2 and Comparative Example 2 were evaluated underthe different test conditions. More specifically, lubricant employed inExample 10 and Comparative Example 10 was lubricant obtained by fullymixing an ester component which was fatty monoglyceride (a maincomponent of fatty acid is stearic acid) of 7 weight % in the total ofthe lubricant with poly-alfa-olefin. The other conditions of Example 10and Comparative Example 10 were the same as those of Example 2. Theevaluation of Example 10 and Comparative Example 10 were also the sameas that of Example 2.

EXAMPLE 11 and COMPARATIVE EXAMPLE 11

The gears of Example 2 and Comparative Example 2 were evaluated underthe different test conditions. More specifically, Lubricant employed inExample 11 and Comparative Example 11 was lubricant obtained by fullymixing an ester component which was butyl stearate of 2 weight % in thetotal of the lubricant with poly-alfa-olefin. The other conditions ofExample 11 and Comparative Example 11 were the same as those of Example2. The evaluation of Example 11 and Comparative Example 11 were also thesame as that of Example 2.

EXAMPLE 12 and COMPARATIVE EXAMPLE 12

Chromium molybdenum steel defined as SCM440H in JIS (Japan IndustrialStandard) was employed as material of the gears of Example 12. Thematerial was machined into gears defined by the followingspecifications, and the carburizing, quenching and tempering processeswere applied to the machined gears. Thereafter, a finishing touch wasapplied to the processed gear. The gears were assembled into a planetarygear mechanism of 2K-H type. In the evaluation of the gears, a sun gearfunctioning as an input gear, a ring gear was fixed, and a carrier ofplanetary gears functioning as an output.

<Specifications of gear>

Type of gear: spur gear

Module: 2 mm

Number of gear teeth of sun gear: 60

Number of gear teeth of planetary gear: 24

Number of gear teeth of ring gear: 144

Number of planetary gears: 3

Face width: 6 mm

The sun gear and the planetary gears of Example 12 were degreased andset in a vacuum chamber. Hard carbon films were formed on tooth surfacesof the gears by arc ion plating (AIP) process in the vacuum chamber. Athickness of the film at a center portion of the sun gear was 1.7 μm.The hydrogen amount in the film was 0.1 atom % as a result of themeasurement using a secondary ion mass spectroscopy (SIMS). The toothsurfaces were polished to remove droplets of the film and to smoothenthe surface. The ring gear was not coated with the hard carbon film.

Using the power circulation type gear test equipment, the total loss ofthe tested gear was measured under a condition that the tested gearswere wholly soaked in poly-alfa-olefin (PAO) oil and that the testedgear was rotated at a speed of 6000 rpm (the revolution speed of the sungear).

On the other hand, the gears of Comparative Example 12 were the same inshape and in material as those of Example 12 except that no hard carbonfilm was formed on the gears of Comparative Example 12. The total lossof Comparative Example 12 was measured under the condition as same asthat of Example 12.

EXAMPLE 13 and COMPARATIVE EXAMPLE 13

The gears of Example 2 and Comparative Example 2 were evaluated underthe different test conditions. More specifically, Lubricant employed inExample 13 and Comparative Example 13 was JIS industrial gear oilspecified by type 2 and viscosity classification ISO VG 220. The otherconditions of Example 13 and Comparative Example 13 were the same asthose of Example 2. The evaluation of Example 13 and Comparative Example13 were also the same as that of Example 2.

EXAMPLE 14 and COMPARATIVE EXAMPLE 14

The gears of Example 2 and Comparative Example 2 were evaluated underthe different test conditions. More specifically, lubricant employed inExample 14 and Comparative Example 14 was lubricant obtained by fullymixing an ester component which was fatty monoglyceride (a maincomponent of fatty acid is stearic acid) of 5 weight % in the total ofthe lubricant with JIS industrial gear oil specified by viscosityclassification ISO VG 220 type 2. The other conditions of Example 14 andComparative Example 14 were the same as those of Example 2. Theevaluation of Example 14 and Comparative Example 14 were also the sameas that of Example 2.

EXAMPLE 15 and COMPARATIVE EXAMPLE 15

The gears of Example 2 and Comparative Example 2 were evaluated underthe different test conditions. More specifically, lubricant employed inExample 15 and Comparative Example 15 was lubricant obtained by fullymixing an ester component which was fatty monoglyceride (a maincomponent of fatty acid is stearic acid) of 4 weight % in the total ofthe lubricant with JIS industrial turbine oil specified by JIS-2213Ktype 2. The other conditions of Example 15 and Comparative Example 15were the same as those of Example 2. The evaluation of Example 15 andComparative Example 15 were -also the same as that of Example 2.

EXAMPLE 16 and COMPARATIVE EXAMPLE 16

The gears of Example 7 and Comparative Example 7 were evaluated underthe different test conditions. More specifically, Lubricant employed inExample 16 and Comparative Example 16 was lubricant obtained by fullymixing an ester component which was fatty monoglyceride (a maincomponent of fatty acid is stearic acid) of 7 weight % withpoly-alfa-olefin, as same as that employed in Examples 10 and 11. Theother conditions of Example 16 and Comparative Example 16 were the sameas those of Example 7. The evaluation of Example 16 and ComparativeExample 16 were also the same as that of Example 7 and ComparativeExample 7.

EXAMPLE 17 and COMPARATIVE EXAMPLE 17

The gears of Example 2 and Comparative Example 2 were evaluated underthe different test conditions. More specifically, lubricant employed inExample 17 and Comparative Example 17 was lubricant obtained by fullymixing an ester component which was fatty diglyceride (a main componentof fatty acid is stearic acid) of 2 weight % with poly-alfa-olefin. Theother conditions of Example 17 and Comparative Example 17 were the sameas those of Example 2. The evaluation of Example 17 and ComparativeExample 17 were also the same as that of Example 2.

EXAMPLE 18 and COMPARATIVE EXAMPLE 18

The gears of Example 2 and Comparative Example 2 were evaluated underthe different test conditions. More specifically, lubricant employed inExample 18 and Comparative Example 18 was lubricant obtained by fullymixing pinacol having two hydroxy groups in one molecular at 0.8 weight% with poly-alfa-olefin. The other conditions of Example 18 andComparative Example 18 were the same as those of Example 2. Theevaluation of Example 18 and Comparative Example 18 were also the sameas that of Example 2.

EXAMPLE 19 and COMPARATIVE EXAMPLE 19

The gears of Example 2 and Comparative Example 2 were evaluated underthe different test conditions. More specifically, lubricant employed inExample 19 and Comparative Example 19 was lubricant obtained by fullymixing 1,10-decandiol of 2.0 weight % and poly-alfa-olefin. The otherconditions of Example 19 and Comparative Example 19 were-the same asthose of Example 2. The evaluation of Example 19 and Comparative Example19 were also the same as that of Example 2.

EXAMPLE 20 and COMPARATIVE EXAMPLE 20

The gears of Example 2 and Comparative Example 2 were evaluated underthe different test conditions. More specifically, lubricant employed inExample 20 and Comparative Example 20 was lubricant obtained by fullymixing dodecyl alcohol of 2.0 weight % with poly-alfa-olefin. The otherconditions of Example 20 and Comparative Example 20 were the same asthose of Example 2. The evaluation of Example 20 and Comparative Example20 were also the same as that of Example 2.

EXAMPLE 21 and COMPARATIVE EXAMPLE 21

The gears of Example 2 and Comparative Example 2 were evaluated underthe different test conditions. More specifically, lubricant employed inExample 21 and Comparative Example 21 was glycerin. No additive wasadded in glycerin. The other conditions of Example 21 and ComparativeExample 21 were the same as those of Example 2. The evaluation ofExample 21 and Comparative Example 21 were also the same as that ofExample 2.

EXAMPLE 22 and COMPARATIVE EXAMPLE 22

The gears of Example 2 and Comparative Example 2 were evaluated underthe different test conditions. More specifically, lubricant employed inExample 22 and Comparative Example 22 was ethylene glycol. No additivewas added in ethylene glycol. The other conditions of Example 22 andComparative Example 22 were the same as those of Example 2. Theevaluation of Example 22 and Comparative Example 22 were also the sameas that of Example 2.

As to Examples 1 through 22 and Comparative Examples I through 22, themeasurement results of the power transmission efficiencies thereof wereshown in Table 1 together with the specifications of the gears, such asmaterial, module, teeth number, film forming method, hydrogen amount inthe hard carbon film, and a kind of lubricant.

As is apparent from Table 1, it was recognized that the gears ofExamples 1 through 22 according to the present invention performedexcellent power transmission efficiencies as compared with ComparativeExamples 1 through 22, respectively. Specifically, Example 21 performedthe excellent power transmission efficiency. Further, Example 16 issubsequently preferable in case that the selection of lubricant islimited such that the lubricant is commonly used in the other slidingpair. In case that both of cost and power transmission efficiency arebalancedly minded, Example 8 through 10 are subsequently preferable.

With the thus arranged gear according to the present invention, by filmthe hard carbon film on al least a part of tooth face of the gear, itbecomes possible to largely decrease the friction of the gear relativeto the counter gear (meshed gear). Further, by employing a specificallyblended lubricant, the friction of the gear is further preferablydecreased. This provides the gear further improved in power transmissionefficiency. Specifically, in case that lubricant may be freely selected,a largely improved power transmission efficiency is obtained by alubricant wherein alcohol group is used as a main component of thelubricant. Further, the gear according to the present invention ispreferably adapted to a mechanism which has a lot of gear meshingportions, such as a planetary gear mechanism and speed reductionmechanism. When the gear is employed in such mechanisms, the powertransmission efficiency is further improved. TABLE 1 Example/ComparativeExample 1 2 3 4 5 6 7 8 Material SCM420H SCM440H SCM420H SCM420H SCM440HSCM420H SCM440H SMC440H Module (mm)  4  2  6  2  2  4  2  2 Number of 6060 120 60 60 60 helical 60 60 Teeth Film Forming AIP AIP AIP SpatteringPlasma AIP AIP (Both AIP Method CVD Gears) H amount  0.1  0.1  0.2  0.125  0.1  0.1  0.1 (atom %) Lubricant PAO PAO PAO PAO PAO PAO PAO PAO +Oleic acid monoglyceride Example 97.5 97.0  96.9 97.7 95.1 96.8 99.098.1 Transmission Efficiency (%) Comp. Example 93.3 93.1  94.2 93.1 93.194.0 93.1 94.4 Transmission Efficiency (%) Example/Comp. Example 9 10 1112 13 14 15 Material SCM440H SCM440H SCM440H SCM440H SCM440H SCM440HSCM440H Module (mm)  2  2  2  2  2  2  2 Number of 60 60 60 60/24/1442K-H 60 60 60 Teeth planetary gear mechanism Film Forming AIP AIP AIPAIP AIP AIP AIP Method H amount  0.1  0.1  0.1  0.1  0.1  0.1  0.1 (atom%) Lubricant PAO + PAO + PAO + Butyl PAO Gear oil Gear oil + Turbineoil + Palmitic acid Stearic acid stearate Stearic acid Stearic acidmonoglyceride monoglyceride (Stearic acid monoglyceride monoglyceridebutyl ester) Example 97.9 98.4 97.7 93.3 95.5 96.7 94.7 TransmissionEfficiency (%) Comp. Example 93.5 94.6 94.4 86.0 92.5 92.2 91.1Transmission Efficiency (%) Example/Comp. Example 16 17 18 19 20 21 22Material SCM440H SCM440H SCM440H SCM440H SCM440H SCM440H SCM440H Module(mm)  2  2  2  2  2  2  2 Number of Teeth 60 60 60 60 60 60 60 FilmForming AIP (Both AIP AIP AIP AIP AIP AIP Method Gears) H amount  0.1 0.1  0.1  0.1  0.1  0.1  0.1 (atom %) Lubricant PAO + Stearic PAO +Stearic PAO + Pinacol PAO + 1,10- PAO + Dodecyl Glycerin Ethylene acidacid decanediol alcohol glycol monoglyceride diglyceride Example 99.297.9 97.1 97.8 97.6 99.3 99.2 Transmission Efficiency (%) Comp. Example94.6 93.2 93.2 93.3 93.7 92.7 91.6 Transmission Efficiency (%)NotesAIP: arc ion plating process, PAO: poly-alfa (α)-olefin.

This application is based on Japanese Patent Applications No.2003-204238 filed on Jul. 31, 2003 and No. 2004-127632 filed on Apr. 23,2004 in Japan. The entire contents of these Japanese Patent Applicationsare incorporated herein by reference.

Although the invention has been described above by reference to certainembodiments of the invention, the invention is not limited to theembodiments described above. Modifications and variations of theembodiments described above will occur to those skilled in the art, inlight of the above teaching. The scope of the invention is defined withreference to the following claims.

1. A gear comprising: a tooth surface; and a hard carbon film formed onat least a part of the tooth surface.
 2. The gear as claimed in claim 1,wherein a hydrogen amount in the hard carbon film is smaller than orequal to 1 atomic percent.
 3. The gear as claimed in claim 2, wherein ahydrogen amount in the hard carbon film is smaller than or equal to 0.3atomic percent.
 4. The gear as claimed in claim 1, wherein the gear isused as a gear of a planetary gear mechanism.
 5. The gear as claimed inclaim 1, wherein the gear is used as a gear of a speed reductionmechanism.
 6. The gear as claimed in claim 1, wherein the gear is usedin a lubricant.
 7. The gear as claimed in claim 6, wherein the lubricantincludes a mixture employing poly-alfa-olefin (PAO) as a base oil. 8.The gear as claimed in claim 6, wherein the lubricant includes anadditive including a hydroxyl group.
 9. The gear as claimed in claim 8,wherein the additive includes two or more hydroxyl groups in a molecule.10. The gear as claimed in claim 6, wherein an additive in the lubricantincludes ester.
 11. The gear as claimed in claim 6, wherein an additivein the lubricant includes monoester of glycerin.
 12. The gear as claimedin claim 6, wherein a main component of the lubricant is ahydroxy-compound.
 13. The gear as claimed in claim 12, wherein thehydroxy-compound includes alcohol.
 14. The gear as claimed in claim 13,wherein the alcohol includes glycerin.
 15. The gear as claimed in claim13, wherein the alcohol includes ethylene glycol.